The invention relates to the fabrication of shaped structures that can be useful in the production of various devices including optical and photonic devices and structural devices.
New applications in multiple industries have driven needs for advances in manufacturing and integration technologies for optics and photonics. Functionality, miniaturization, and lower costs are nearly universal drivers for communications, sensors, biomedical applications, data storage, and other industries. These factors drive parallel needs for technical innovation and scalable, cost-efficient manufacturing. These drivers present significant research challenges and opportunities for further research in photonic components, systems, materials, and manufacturing.
Diffractive, refractive, and guided-wave micro-optics have become core technologies for individual components and within integrated optical microsystems. Both free-space and guided-wave optics offer a great deal of promise due to broad functionality and the potential for wafer-level manufacturing using techniques leveraged from the microelectronics industry. Achievable functions from both free-space and guided wave optics are ultimately defined by structure geometry and optical material properties.
Diffractive optics, refractive optics, sub-wavelength optics, photonic crystals, and optical waveguides fall under the general headings of micro-optics or nano-optics. As in the semiconductor industry, there is an ongoing push to manufacture smaller structures. For example, a variety of optical functions have been realized through fabrication of effective media with sub-wavelength structures to engineer a locally effective refractive index in the optical material.
Photonic crystals are another class of devices receiving a great deal of interest due to the wide range of functions that can be realized through fabrication on scales at or below the wavelength of light. These periodic structure exhibit a frequency range over which light propagation is forbidden, a property referred to as the photonic bandgap. One, two, and three-dimensional crystals have been created to implement functions such as optical circuits, optical add-drop functions, resonator cavities, slow light and wavelength conversion. Depending on the geometry, the device may perform more like a free-space component, and in other cases like a guided wave element. In nearly all cases, however, the components are challenging to fabricate, and even more difficult to manufacture, particularly for use at infrared or visible wavelengths.
Over the past decade, multiple examples of integration of passive micro-optics with active devices (MEMS, laser sources, detectors, etc.) have been demonstrated. Both hybrid and monolithic approaches have been used for integration of optical deices. The trend towards monolithic functional integration continues to accelerate due to the need for performance, new functionality, miniaturization, and cost reduction.
Metamaterials are materials that are structured at the micro- or nano-scale in order to give the composite useful properties or performance unrealizable from the homogeneous bulk. The results are materials that exhibit optical properties not observed in nature. The effective medium and photonic devices previously discussed may be considered metamaterials. Photonic crystals in particular have been the subject of significant international research due to their potential for manipulating light in ways that can not be achieved using homogeneous materials. The ability to form photonic crystals into non-planar shapes could enable, for example, the conversion of a complex unusable output mode from an intracavity source into a well-conditioned, quasi-plane wave. V. Berger, “From photonic band gaps to refractive index engineering,” Opt. Materials, vol. 11, pp. 131-142, 1999. It is noted that structures in the Berger article are conceptional only and Berger does not describe the actual fabrication of such devices. Photonic crystal structures require dimensional tolerances of less than about 10 nm in many cases. Previous methods of producing photonic crystals typically do not achieve such tight tolerances.
Integrating and interfacing discrete nanoscale devices with components and systems that may be several orders of magnitude larger presents significant challenges. Similarly, ordering of the particles, proximity to other nanoparticles, and surrounding materials may alter their performance.
There are multiple techniques with the potential for large scale volume manufacturing of optical nanostructures. Lithography using holographic interference of multiple laser beams can be used, but this technique by itself offers limited flexibility for fabricating different structures. Minimum feature sizes below about 100 nm may be achievable using direct patterning with electron beam lithography, or modern step-and-repeat or step-and-scan projection lithography systems (“steppers”) using resolution enhancing methods such as phase shifting masks, customized illumination schemes, immersion lithography, and extreme ultraviolet lithography. Each of these approaches typically requires expensive, complex equipment and processes to manufacture nano-structures. The serial nature of e-beam lithography typically renders the technique prohibitively expensive for anything more than a few components. Existing monolithic integration techniques often require similar manufacturing infrastructure, while hybrid integration requires additional equipment and processing steps, usually through serial assembly.
Phase masks have been previously described for synthesis of 3D light fields and the use of phase masks for fabrication of three-dimensional structures has been demonstrated in simple polymers. See R. Piestun and J. Shamir, “Synthesis of Three-Dimensional Light Fields and Applications,” Proc. of the IEEE, vol. 90, pp. 222-244, 2002, which discusses light field generation; see also Divliansky, T. S. Mayer, K. S. Holliday, and V. H. Crespi, “Fabrication of three-dimensional polymer photonic crystal structures using single diffraction element interference lithography,” Appl. Phys. Lett., vol. 82, pp. 1667-1669, 2003; S. Jeon, J.-U. Park, R. Cirelli, S. Yang, C. E. Heitzman, P. V. Braun, P. J. A. Kenis, and J. A. Rogers, “Fabricating complex three-dimensional nanostructures with high-resolution conformable phase masks,” PNAS, vol. 101, pp. 12428-12433, 2004, which discuss the fabrication of 3D structures with planar surfaces.
The majority of the work in three dimensional structured micro- and nano-particles has focused on the creation of 3D macroporous lattices from micro or nanoparticles through self assembly. Quantum dots as small as about 4 nm have been used to assemble theses structures, though typically spherical particles from 50 nm to several microns in diameter are used. In some cases, electric fields have been used to aid in the arrangement of the particles. These opal or inverse opal lattices (i.e., where the empty spaces in the lattices are infiltrated with a material and the original spheres are dissolved away) are usually used as 3D photonic crystals, and little attention has been given to manufacturing or to forming the lattice into larger structures of specific geometries.
A typical assembly method utilizes polyurethane packing cells to form 3D lattices of mesocopic colloids. These homogeneous lattices were developed over an area of 0.5 cm2, though the assembly process typically requires a couple of days. B. T. Mayers, B. Gates, and Y. Xia, “Crystallization of Mesoscopic Colloids into 3D Opaline Lattices in Packing Cells Fabricated by Replica Molding,” Adv. Mater., vol. 12, pp. 1629-1632, 2000. In this method, the shape of the mold is very simple and not a direct factor in the optical performance of the fabricated component. Structured nano-patterning has also been performed using dip-pen writing with nanoparticle inks as serial, single point processes for forming structural arrangements of nanoparticles.
Methods of soft lithography have been demonstrated as a means to pattern hybrid composite materials. For example, researchers utilized soft lithography to print electronic inks formed by incorporating nanoparticles into a host matrix. G. Blanchet and J. A. Rogers, “Printing Techniques for Plastic Electronics,” J. Imaging Science and Tech., vol. 47, pp. 296-303, 2003. Microlens arrays have been molded using photopolymers loaded with functionalized silica nanoparticles to increase the structural stability of the polymer. M. V. Kunnavakkam, F. M. Houlihan, M. Schlax, J. A. Liddle, P. Kolodner, O. Nalamasu, and J. A. Rogers, “Low-cost, low-loss microlens array fabricated by soft-lithography replication process,” Appl. Phys. Lett., vol. 82, pp. 1152-1154, 2003. Researchers also demonstrated the creation of nanocomposite optical gain media and presented methods for synthesis of polymer and solgel nanocomposites.
None of the previously described synthetic nanofabrication methods provide commercially viable, scalable paths for 3D nanofabrication. Similarly, none of the previously described synthetic nanofabrication methods provide a method in which the shape of the nanocomposite structure plays an integral role in the overall functionality of the resulting device.